[NFC][SimplifyCFG] 'Safely extract low bits' pattern will also benefit from -phi-node-folding-threshold=3

This is the naive implementation of x86 BZHI/BEXTR instruction:
it takes input and bit count, and extracts low nbits up to bit width.
I.e. unlike shift it does not have any UB when nbits >= bitwidth.
Which means we don't need a while PHI here, simple select will do.
And if it's a select, it should then be trivial to fix codegen
to select it to BEXTR/BZHI.

See https://bugs.llvm.org/show_bug.cgi?id=34704

llvm-svn: 370369

1 files changed, 35 insertions, 0 deletions

diff --git a/llvm/test/Transforms/SimplifyCFG/safe-low-bit-extract.ll b/llvm/test/Transforms/SimplifyCFG/safe-low-bit-extract.ll
new file mode 100644
index 00000000000..354cfff7ba7
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/safe-low-bit-extract.ll
@@ -0,0 +1,35 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+; This is the naive implementation of x86 BZHI/BEXTR instruction:
+; it takes input and bit count, and extracts low nbits up to bit width.
+; I.e. unlike shift it does not have any UB when nbits >= bitwidth.
+; Which means we don't need a while PHI here, simple select will do.
+define i32 @extract_low_bits(i32 %input, i32 %nbits) {
+; CHECK-LABEL: @extract_low_bits(
+; CHECK-NEXT:  begin:
+; CHECK-NEXT:    [[SHOULD_MASK:%.*]] = icmp ult i32 [[NBITS:%.*]], 32
+; CHECK-NEXT:    br i1 [[SHOULD_MASK]], label [[PERFORM_MASKING:%.*]], label [[END:%.*]]
+; CHECK:       perform_masking:
+; CHECK-NEXT:    [[MASK_NOT:%.*]] = shl nsw i32 -1, [[NBITS]]
+; CHECK-NEXT:    [[MASK:%.*]] = xor i32 [[MASK_NOT]], -1
+; CHECK-NEXT:    [[MASKED:%.*]] = and i32 [[MASK]], [[INPUT:%.*]]
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    [[RES:%.*]] = phi i32 [ [[MASKED]], [[PERFORM_MASKING]] ], [ [[INPUT]], [[BEGIN:%.*]] ]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+begin:
+  %should_mask = icmp ult i32 %nbits, 32
+  br i1 %should_mask, label %perform_masking, label %end
+
+perform_masking: ; preds = %begin
+  %mask.not = shl nsw i32 -1, %nbits
+  %mask = xor i32 %mask.not, -1
+  %masked = and i32 %mask, %input
+  br label %end
+
+end:             ; preds = %perform_masking, %begin
+  %res = phi i32 [ %masked, %perform_masking ], [ %input, %begin ]
+  ret i32 %res
+}